1. Field of the Invention
This invention relates to a method of producing an extra-low iron loss grain oriented silicon steel sheet, and more particularly to conduct refinement of magnetic domains and hence advantageous improvement of iron loss properties by subjecting a coating layer to EB (electronic beam) radiation. The coating may be formed after finish annealing or applied to a mirror finished steel sheet surface after finish annealing to a CVD, ion plating or iron implantation process with nitride, carbide, oxide or the like, and comprises forming an insulation coating on the resulting tension layer and then subjecting the insulation coating to electron beam (EB) irradiation in a direction extending across the rolling direction.
2. Related Art Statement
Lately, remarkable developments and efforts for satisfying the improvement of electrical and magnetic properties in grain oriented silicon steels, particularly ultimate demand on reduction of iron loss are gradually producing good results.
As is well-known, the grain oriented silicon steel sheet, wherein secondary recrystallized grains are highly aligned in {110}&lt;001&gt; orientation, namely Goss orientation, is mainly used as a core for transformer and other electrical machinery and equipment. In this case, it is required that the magnetic flux density (represented by B.sub.10 value) is high, and the iron loss (represented by W.sub.17/50 value) is low.
Since these grain oriented silicon steel sheets are usually manufactured through many complicated steps, a great of inventions and improvements are applied to the above steps, whereby low iron loss grain oriented silicon steel sheets having B.sub.10 of not less than 1.90 T and W.sub.17/50 of not more than 1.05 W/kg when the product thickness is 0.30 mm or B.sub.10 of not less than 1.89 T and W.sub.17/50 of not more than 0.90 W/kg when the product thickness is 0.23 mm are manufactured up to the present.
Lately, supreme demands on the reduction of power loss become considerable in view of energy-saving. Particularly, a system of "Loss Evaluation" wherein the reduction percentage of iron loss is converted into a money to load on the cost of the transformer in the manufacture of low loss transformer is widely spread in Europe and America.
Under the above circumstances, there has recently been proposed a method wherein local microstrain is introduced into the surface of the grain oriented silicon steel sheet by irradiating a laser beam onto the steel sheet surface in a direction substantially perpendicular to the rolling direction after finish annealing to thereby conduct refinement of magnetic domains and hence reduce iron loss (Japanese Patent Application Publication Nos. 57-2,252, 57-53,419, 58-26,405 and 58-26,406).
Such a magnetic domain refinement is effective for grain oriented silicon steel sheet not subjected to strain relief annealing, as is done in the manufacture of stacked lamination-core type transformers. However, in the case of wound-core type transformers, strain relief annealing is performed after the magnetic domain refinement, so that the local microstrain intentionally produced by laser irradiation is released by annealing to make the width of magnetic domains wide, and consequently the laser irradiating effect is lost.
On the other hand, Japanese Patent Application Publication No. 52-24,499 discloses a method of producing an extra-low iron loss grain oriented silicon steel sheet wherein the surface of the grain oriented silicon steel sheet is subjected to mirror finishing after finish annealing or a metal plating is applied to the mirror finished surface, or further an insulation coating is baked thereon.
However, the mirror finishing for improving the iron loss does not sufficiently contribute to the reduction of iron loss in comparison with the remarkable cost-increase of the manufacturing step. Particularly, there is a problem on the adhesion property to the insulation coating indispensably applied and baked after mirror finishing. Therefore, such mirror finishing is not yet used in the present manufacturing step.
Further, there is proposed a method, wherein the steel sheet surface is subjected to mirror finishing and then a thin coat of oxide ceramics is deposited thereon, in Japanese Patent Application Publication No. 56-4,150. In this method, however, the ceramic cost tends to peel off the steel sheet surface when subjecting it to high temperature annealing above 600.degree. C., so that it can not be adopted in the actual manufacturing step.
Moreover, Japanese Patent laid open No. 59-229,419 proposes a method wherein heat energy is locally applied to the surface of the silicon steel sheet to form a heat strain zone. However, the effect based on the preferential formation of such a local heat strain zone is lost by high temperature annealing above 600.degree. C. In addition, there is proposed a method of introducing an artificial grain boundary into silicon steel sheet having a secondary grain size of not less than 3 mm in Japanese Patent laid open No. 58-144,424 and a method of irradiating plasma flame to grain oriented silicon steel sheet after finish annealing in Japanese Patent laid open No. 62-96,617. In the latter methods, however, the effect is lost in case of material for wound-core type transformers subjected to the strain relief annealing.